News Release

Woolly Bear Caterpillars and Weather Predictions

This article was originally published on October 3, 2011 and expired on October 10, 2011. It is provided here for archival purposes and may contain dated information.

Have you noticed all the woolly bear caterpillars lately? Rhonda Ferree, Horticulture Educator with University of Illinois Extension, has noticed numerous woolly bear caterpillars crossing roadways this fall.

Woolly bear caterpillars are about 2-inches long and covered with stiff bristles. They are black with a broad band of red-brown bristles around the middle. These caterpillars are the larval stage of the Isabella moth.

These caterpillars feed on mostly wild herbaceous plants such as lambsquarter. Although they seldom attack desirable plants, if they strip the weeds of foliage, they may move on to flowers and other landscape plants. Once they move onto desired plants, they usually are too large to be effectively controlled. In some cases, handpicking the larvae off of the plants may be an option, but in most cases trying to control these caterpillars this late in the season is not reasonable.

According to superstition, the amount of black on the woolly bear's bristle coating forecasts the severity of the coming winter. Ferree says, "It is the relative proportions of the black and reddish-brown portions of the caterpillar that are supposed to predict the winter." The longer the black segments on the ends of the caterpillar, the harsher the coming winter.

One of the problems with trying to forecast the winter using these insects is that the tiger moth has similar caterpillars as its larval stage. Unfortunately, there are about 260 species of the tiger moth and each species has a different color variation. Plus the caterpillars shed their skins or molt six times before reaching adult size, and their colors change with each molt.

According to Donald Lewis, Entomologist from Iowa State University, there is some year-to-year variation in the amount of black hair on these caterpillars, but the differences are caused by age and wetness. Older caterpillars have more black than young ones and caterpillars that fed and grew in an area where the fall weather was wetter have more black hair than caterpillars from dry areas.

So why do the woolly bears cross the road? No one really knows why, but they cross roads and paths on warm days in late fall. Some people even believe that this can predict the weather. If they are going south, it is going to be a harsh winter. If they are headed north, it will be a mild winter. Rhonda was driving east and west, so she doesn't know what that means!

If you don't believe woolly bears can predict the weather, you might instead want to look at pig spleens, groundhogs, hornets, persimmon seeds, or read what "The Old Farmer's Almanac" says. You can watch the weather forecasters using their high-tech equipment. Or, you can just wait and see what winter has in store for us. Rhonda suggests that you enjoy a beautiful fall day with a nice walk outside while you wait.